Color measuring apparatus

ABSTRACT

A color measuring apparatus includes a measurement assembly which includes at least one illumination assembly for applying substantially parallel illumination light to a measurement spot of a measurement object and a pick-up assembly for capturing the measurement light radiated back from the measurement spot in an observation direction and for converting the same into corresponding electrical signals. The illumination assembly includes at least two illumination subassemblies which illuminate the measurement spot from different illumination sub-directions near a first preset nominal illumination direction, each with preferably parallel illumination light. By the illumination from different illumination sub-directions slightly deviating from the nominal illumination direction, angular errors of the illumination assembly can be compensated for in a simple manner.

BACKGROUND 1. Technical Field

The present disclosure relates generally to color measuring apparatuswith a measurement assembly which includes at least one illuminationassembly for applying preferably substantially parallel illuminationlight to a measurement spot of a measurement object and a pick-upassembly for capturing the measurement light radiated back from themeasurement spot and for converting the same into correspondingelectrical signals. The disclosed color measuring apparatus can beformed as autonomous apparatus or as measurement periphery foremployment in connection with a controlling computer evaluatingmeasurement data independently of the underlying measurement technology.Autonomous color measuring apparatus generally include all of thecontrol and display organs required for the measurement operation aswell as their own current supply and moreover are frequently alsoequipped with an interface for communication with a computer, whereinboth measurement data and control data can be exchanged with thecomputer. Color measuring apparatus configured as measurement peripheryusually do not have their own control and display organs and are—likeeach other computer peripheral—controlled by a superordinated computer.For communication with a computer, modern color measuring apparatus areoften, e.g., fitted with a so-called USB (Universal Serial Bus)interface, via which the current supply can also be effected at the sametime in many cases (from the connected computer).

2. Background Art

Nowadays, metallic colors and varnishes with effect pigments are moreand more employed not only in the automotive industry. Such colorsexhibit a severe angular dependency. Varnishes with aluminum flakes forexample exhibit a severe brightness flop. Varnishes with interferenceeffect pigments additionally also exhibit color differences with variedobservation or illumination direction. For measuring such varnishes,multi-angle measuring apparatus have been established. The glossmeasurement is a related issue, in which the measurement result is alsosensitive to angle.

Measuring apparatus which are able to acquire such characteristics haveto be adapted to illuminate the measurement object in one or moredifferent, exactly defined illumination directions (nominal illuminationdirections) and to pick up the light radiated back from the measurementobject from at least one exactly defined observation direction (nominalobservation direction). Observation direction and illumination directioncan be exchanged. Color measuring apparatus of this type are, e.g.,described in great detail in the documents EP 2 703 789 A1 and EP 2 728342 A1.

In the publication “Geräteprofilierung: Management globalerFarbkonsistenz” of Wilhelm H. Kettler, DFO Tagung Qualitätssicherung undPrüfverfahren, 2008, various causes are set out, which can result inmeasurement errors in the employment of such color measuring apparatus.In particular, the so-called systematic errors, which are attributableto certain apparatus imperfections such as erroneous calibration belongthereto. In a lecture delivered by Wilhelm H. Kettler for Farbe undLack/Seminare Modul 2: Tiefere Einblicke in die Farbmetrik Jun. 25-27,2014, Stuttgart (FPL) with the title “Farbmanagement”, the so-calledangular errors are in particular also indicated, which can arise by thegeometric conditions of the illumination and observation beam paths aswell as by the apertures of the illumination and observation beam paths.Angular errors especially effect particularly severely in themeasurement on samples with effect colors.

The present invention primarily deals with avoiding and compensating formeasurement errors caused by such angular errors, respectively.

Therefore, by the present invention, a color measuring apparatus of thegeneric type is to be improved to the effect that angular errors can besimply corrected such that the illumination and observation directionspreset by the respective measurement geometry are exactly complied withand thereby measurement value corruptions are avoided.

This object underlying the invention is equally solved by the colormeasuring apparatus according to the invention characterized by thefeatures of the independent claim 1 or by the features of theindependent claim 14. Advantageous configurations and developments ofthe color measuring apparatus according to the invention are the subjectmatter of the dependent claims.

SUMMARY

The nature of the first form of realization of the invention is in thefollowing: a color measuring apparatus has a measurement assemblyincluding at least one illumination assembly for applying substantiallyparallel illumination light to a measurement spot of a measurementobject and a pick-up assembly for capturing the measurement lightradiated back from the measurement spot in an observation direction andfor converting the same into corresponding electrical signals. Theillumination assembly includes at least two illumination subassembliesilluminating the measurement spot from different illuminationsub-directions located near a first preset nominal illuminationdirection each with parallel illumination light. Advantageously, but notnecessarily, therein, the illumination sub-directions enclose the firstnominal illumination direction between them.

By the division of the illumination assembly in two or more illuminationsubassemblies, measurement errors caused by misalignment of theillumination assembly can be compensated for or avoided.

Conveniently, the illumination sub-directions are angularly offset toeach other by an angle of maximally 4° to 8°.

Advantageously, the illumination assembly includes maximally five toseven illumination subassemblies, which illuminate the measurement spotfrom different illumination sub-directions near to each other andtherein preferably, but not necessarily, enclose a nominal illuminationdirection between them.

According to an advantageous embodiment, the color measuring apparatushas at least one further illumination assembly, which illuminates themeasurement spot substantially from a further preset nominalillumination direction different with respect to the first presetnominal illumination direction with parallel illumination light.

Therein, the at least one further illumination assembly advantageouslyincludes at least two illumination subassemblies illuminating themeasurement spot from different illumination sub-directions near thefurther preset nominal illumination direction each with parallelillumination light.

Conveniently, the illumination subassemblies each have a light source,which is composed of at least one, preferably two or more light emittingdiodes with different emission spectra.

According to a further advantageous aspect of the invention, the colormeasuring apparatus has an electronic control for the illuminationsubassemblies and the pick-up assembly and the control is formed toseparately control the illumination subassemblies within each oneillumination assembly.

Advantageously, therein, the control is formed to individually controlthe activation durations of the illumination subassemblies each withinone illumination assembly.

According to a further advantageous embodiment, the control is formed toactivate the illumination assemblies or the illumination subassembliesthereof and the pick-up assembly for at least two differently longmeasurement durations.

Very particularly advantageously, the color measuring apparatus hasmeans for calculating derivatives of the reflection factors for anominal illumination angle for angular errors in the direction of themeasurement plane.

The nature of the second form of realization of the color measuringapparatus according to the invention is in the following: a colormeasuring apparatus includes a measurement assembly including at leastone illumination assembly for applying preferably substantially parallelillumination light to a measurement spot of a measurement object and atleast one pick-up assembly for capturing the measurement light radiatedback from the measurement spot and for converting the same intocorresponding electrical signals. The at least one pick-up assembly inturn includes at least two pick-up subassemblies, which collectmeasurement light radiated back from the measurement spot from differentobservation sub-directions near a preset nominal observation direction.Advantageously, but not necessarily, therein, the observationsub-directions enclose the nominal observation direction between them.

By the division of the pick-up assembly in two or more pick-upsubassemblies, measurement errors caused by misalignment of the pick-upassembly can be compensated for or avoided.

In the second form of realization of the invention, the roles ofillumination of the measurement object and collection of the measurementlight radiated back from the measurement object are exchanged, but thebasic idea of the invention is obviously the same. Accordingly, the sameanalogously also applies to the number and orientation of the pick-upsubassemblies as the facts mentioned to the illumination subassemblies.The mentioned control of the color measuring apparatus iscorrespondingly adapted to the control of the pick-up subassemblies inanalogous manner.

Additional features, functions and benefits of the present disclosurewill be apparent from the detailed description which follows,particularly when read in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention is explained in more detail based on thefollowing figures.

FIG. 1 shows a slightly simplified illustration of the basicconstruction of the first form of realization of the color measuringapparatus according to the invention;

FIG. 2 shows a schematized illustration of the optical concept of thecolor measuring apparatus according to the invention of FIG. 1.

FIG. 3 shows a schematic illustration of an illumination assembly of thecolor measuring apparatus of FIG. 2,

FIG. 4 shows the beam paths of the illumination assembly of FIG. 3 in aside view,

FIG. 5 shows the beam paths in a floor plan,

FIG. 6 shows a diagram for explaining the principal measurementoperation,

FIG. 7 shows a block diagram of the electronic control of the colormeasuring apparatus,

FIG. 8 shows a schematic illustration of a variant of the illuminationassembly of the color measuring apparatus of FIG. 2,

FIGS. 9, 10 show two schematic sketches for explaining an aperturecorrection, and

FIG. 11 shows a severely simplified schematic illustration of anembodiment of the second form of realization of the color measuringapparatus according to the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

For the following description of figures, the following determinationapplies: if individual reference characters are not registered in afigure, thus, reference is made to the remaining figures and theassociated description parts in this respect. By “measurement assembly”,the entirety of those components of the color measuring apparatus isunderstood, which serve for illumination of a measurement spot on thesurface of a measurement object and for capturing the light radiatedback from this measurement spot and for converting the same intocorresponding electrical signals. By “apparatus normal”, an (imagined)line fixed to apparatus is to be understood, which (ideally) isperpendicular to the surface of the measurement object and defines thecenter of the measurement spot in practical employment of the colormeasuring apparatus. By “illumination direction”, the direction is to beunderstood, in which the measurement spot is illuminated. Analogously,by “observation direction”, the direction is to be understood, fromwhich the measurement light radiated back from the measurement spot ispicked up. By “nominal illumination directions” and “nominal observationdirection”, those illumination and observation directions, respectively,are to be understood, for which the color measuring apparatus isconfigured corresponding to its underlying measurement geometry. Theactual illumination and observation directions can (slightly) deviatefrom the nominal illumination and observation directions e.g. due tomanufacturing tolerances. By “specular direction”, the nominalobservation direction reflected on the surface of the (flat) measurementobject is to be understood. Conventionally, the nominal illumination andobservation directions are related to this specular direction. Amulti-angle color measuring apparatus has multiple nominal illuminationdirections (and optionally also multiple nominal observationdirections). By “measurement plane”, a plane extending through theapparatus normal and all nominal illumination directions and the nominalobservation direction as well as the specular direction is to beunderstood. All of the nominal angular specifications relate todirections located within the measurement plane.

In its general conception, the color measuring apparatus according tothe invention is mostly similarly constructed as the apparatus disclosedin the initially mentioned documents EP 2 703 789 A1 and EP 2 728 342A1. It includes a housing accommodating a measurement assembly and anelectronic control. On the front side of the housing, a display assemblyis provided. Further, control organs are disposed at the top of thehousing. Laterally on the housing, there is an interface (preferablyUSB) for connecting the apparatus to an external computer. The housinghas a measurement opening at the bottom, through which illuminationlight can exit the housing interior and inversely measurement light canenter the housing interior from the outside.

The basic formation of the measurement assembly located in the housingis apparent from FIG. 1. The measurement assembly overall denoted by MAincludes a curved body 11 stationarily fixed in the housing, in whichall of the optical and photo-electrical components of the measurementassembly MA, respectively, are disposed in four continuous chambers 12,13, 14 and 15 in the shown embodiment. In the shown embodiment, thesecomponents are composed of three illumination assemblies 20, 30 and 40and a pick-up assembly 50 with a spectrometer 53, to which themeasurement light is supplied via a lens 51 and a light guide 52. Thespectrometer 53 itself is located outside of the chamber 15. A lens 21,31 and 41, respectively, is each associated with the illuminationassemblies 20, 30 and 40. The three illumination assemblies 20, 30 and40 illuminate a measurement spot MF on a measurement object MO via theassociated lenses 21, 31 and 41 each with parallel radiation beams. Theillumination assemblies 20, 30 and 40 are (except for slight tolerancedeviations) each aligned with a preset nominal illumination direction 2,3 and 4, respectively. The pick-up assembly is aligned with a presetnominal observation direction 5. The entire measurement assembly MA isarranged such that the nominal illumination directions and the nominalobservation direction are in a common measurement plane MP, which alsoincludes an apparatus normal denoted by 0. Further, the measurementplane also includes a specular direction 1, from which the angularpositions of the nominal observation directions 2, 3 and 4 and thenominal observation direction 5 are measured as the reference direction.The illustrated embodiment has a measurement geometry, in which thethree nominal observation directions 2, 3 and 4 extend at an angle of15°, 45° and 110°, respectively, to the specular direction 1, whereinthe second nominal illumination direction 3 coincides with the apparatusnormal 0. The nominal observation direction 5 extends at an angle of 90°to the specular direction 1.

The lenses 21, 31, 41 and 51 can also be completely or partiallyomitted. Similarly, the illumination with parallel light is notmandatory.

In the shown embodiment, the illumination and pick-up beam paths arerectilinearly formed. However, it is also possible, e.g. for reasons ofspace, to fold one or more of the beam paths, thus to redirect them e.g.by means of mirror. It is only essential that the optical axes of theoptical beam sections, which immediately lead to or from the measurementspot are in a common measurement plane.

The illumination assemblies 20, 30 and 40 are controlled by thecomputer-based control 100 (FIG. 7). The latter also controls thepick-up assembly 50 or the spectrometer 53 thereof and processes themeasurement signals thereof. The control 100 can display measurementresults on the display assembly and receive control commands from thecontrol organs. Further, it can communicate with an external computervia the mentioned interface, in particular transmit measurement data andreceive commands and control data. More details are explained furtherbelow in association with FIG. 7.

So far, the described color measuring apparatus basically corresponds tothe color measuring apparatus of this type disclosed in the documents EP2 703 789 A1 and EP 2 728 342 A1 such that the expert does not needfurther explanation in respect thereto.

The present invention does not deal with the basic measurementtechnology as such and the evaluation of the measurement results, butwith the problems of the corruption of the measurement results caused byorientation or angular errors in particular of the illuminationassemblies. In the following, this and the elimination of orcompensation for such measurement corruptions according to theinvention, respectively, is elaborated in more detail based on FIGS.2-5.

According to the most essential basic idea of the invention, at leastone of the illumination assemblies 20, 30 and 40 is composed of two ormore illumination subassemblies 20 a, 20 b, 30 a, 30 band 40 a, 40 b,respectively, which illuminate the measurement field MF of themeasurement object MO in various illumination sub-directions 2 a, 2 b, 3a, 3 band 4 a, 4 b, respectively, relatively slightly deviating from therespective nominal illumination direction 2, 3 and 4, respectively. Inthe embodiment of FIGS. 2-5, each illumination assembly 20, 30 and 40each includes two illumination subassemblies 20 a, 20 b, 30 a, 30 b and40 a, 40 b, which each cast a parallel radiation beam to the measurementspot MF of the measurement object MO via the respectively common lenses21, 31 and 41. Therein, the illumination subassemblies are positionedsuch that the one illumination subassembly 20 a, 30 a and 40 arespectively illuminates the measurement spot MF in an illuminationsub-direction 2 a, 3 a and 4 a, respectively, which deviates in the onedirection from the respective nominal illumination direction 2, 3 and 4,respectively, by a small angle of −Δε/2, and the other illuminationsubassembly 20 b, 30 b and 40 b respectively illuminates the measurementspot MF in an illumination sub-direction 2 b, 3 b and 4 b, respectively,which deviates in the other direction from the respective nominalillumination direction 2, 3 and 4, respectively by a small angle of+Δε/2. Thus, within an illumination assembly 20, 30 and 40,respectively, the respective illumination sub-directions 2 a, 2 b , 3 a,3 b and 4 a, 4 b, respectively, enclose the respective nominalillumination directions 2, 3, and 4, respectively, between them, whereinthe nominal illumination directions do not necessarily have to belocated centrally between the respective illumination sub-directions,but can also asymmetrically divide the angular interval Δε between thetwo illumination sub-directions. The angular interval or thedifferential angle Δε between the two illumination sub-directions 2 a, 2b, 3 a, 3 b, 4 a, 4 b within an illumination assembly 20, 30 and 40,respectively, is comparatively small in proportion to the angles betweenthe nominal illumination directions 2, 3 and 4 and preferably is notmore than 4°-8°, thus ±2° to ±4° with symmetrical deviation from therespective nominal illumination direction 2, 3 and 4, respectively. Foravoiding drawing overload, the angular interval or the differentialangle Δε is only registered in the illumination assembly 20 in FIG. 2.

For the sake of simplicity, the following explanations are effected onlyin connection with the illumination assembly 20. However, they alsoapply in analogous manner to the two other illumination assemblies 30and 40.

FIG. 3 shows the construction of the illumination assembly 20. Itincludes the two already mentioned illumination subassemblies 20 a and20 b next to each other in low distance in the measurement plane denotedby MP. Each of the two illumination subassemblies 20 a and 20 b includesa light source, which is respectively composed of two light emittingdiodes (LEDs) 22 a, 23 a and 22 b, 23 b, respectively, in the examplehere. The two light emitting diodes 22 a , 23 a and 22 b, 23 b,respectively, of the light sources are disposed next to each other inthe direction perpendicular to the measurement plane MP, thus areangularly offset—measured in a projection to the measurement plane—bythe same amount with respect to the nominal illumination direction 2.

The number of the light emitting diodes per light source depends on thespectral range to be measured and the spectral characteristics of thelight emitting diodes. In the extreme case, a single (white) lightemitting diode can be sufficient, in other cases of application,multiple light emitting diodes can also be required, in particular e.g.also UV light emitting diodes. However, one white light emitting diode22 a and 22 b, respectively, and one blue light emitting diode 23 a and23 b, respectively, are generally respectively sufficient.

FIGS. 4 and 5 illustrate the illumination conditions with the twoillumination subassemblies 20 a and 20 b. The two illuminationsubassemblies 20 a and 20 b not illustrated in these figures generate(in combination with the lens 21 commonly associated with them) twoparallel radiation beams 2 as and 2bs in the direction of the respectiveillumination sub-direction 2 a and 2 b, respectively. The illuminationpattern arising on the measurement object is apparent from FIG. 5.

The two radiation beams 2 as and 2 bs overlap in the area of themeasurement spot MF and illuminate the latter. From the measurement spotMF, one beam 5 s of the measurement light radiated back enters thepick-up assembly 50 not illustrated in FIGS. 4 and 5 in observationdirection 5.

From the above explanations, it is clear that the one illuminationsubassembly 20 a illuminates the measurement spot MF at a too smallangle with respect to the nominal illumination direction 2 and the otherillumination subassembly 20 b illuminates the measurement spot MF at atoo large angle with respect to the nominal illumination direction 2.Therefore, the measurement results (spectral reflectivities) achieved inthese two illumination sub-directions will usually deviate in both casesfrom those measurement results, which would be achieved withillumination exactly in the direction of the nominal illuminationdirection. Here, the main idea of the invention now applies: by aweighted mixture of the measurement results with illumination in the twoillumination sub-directions 2 a and 2 b, measurement results can beachieved, which exactly coincide with those measurement results, whichwould be achieved with illumination exactly in the direction of thenominal illumination direction. Therein, it does not play any role howexactly the illumination assembly 20 or the remaining illuminationassemblies 30 and 40 are oriented relative to their respective nominalillumination directions 2 and 3 and 4, respectively, if the latter areonly respectively within the angular intervals Δε between theillumination sub-directions 2 a, 2 b, 3 a, 3 b and 4 a, 4 b,respectively.

If r_(a)(i, λ) is the reflectivity measured with illumination inillumination sub-direction 2 a, 3 a and 4 a, respectively, for thewavelength λ, wherein i stands for one of the three illuminationassemblies 20, 30 and 40, and if r_(b)(i, λ) is the reflectivitymeasured with illumination in illumination sub-direction 2 a, 3 a and 4a, respectively, for the wavelength λ, wherein i again stands for one ofthe three illumination assemblies 20, 30 and 40, then, the correctedreflectivity r_(k)(i, λ) results computationally for the wavelength λaccording to the formula:

r _(k)(i, λ)=g _(i) *r _(a)(i, λ)+(1−g _(i))*r _(b)(i, λ)

wherein g_(i) is a weighting factor from 0 to 1 empirically determinedseparately for each illumination assembly. With a weighting factorapproaching 0, the corrected reflectivity r_(k)(i, λ) will approach thereflectivity r_(b)(i, λ) measured in the illumination sub-direction 2 b,3 b and 4 b, respectively, and with a weighting factor approaching 1,the corrected reflectivity r_(k)(i, λ) will approach the reflectivityr_(a)(i, λ) measured in the illumination sub-direction 2 a, 3 aand 4 a,respectively. The weighting factor g_(i) thus effects an angulardisplacement of the metrologically effective illumination directionwithin the angular interval Δε between the respective two illuminationsub-directions 2 a, 2 b, 3 a, 3 b and 4 a, 4 b, respectively.

In a particular implementation of the invention, the weighting factorg_(i) can also be determined separately for each wavelength λ. In afurther implementation, the weighting factor can also be outside of therange 0 to 1. Generally, the interpolation or extrapolation can also beperformed for more than two sampling points as long as the sum of theweights for the individual spectra is normalized. The above explainedcalculation requires two separated measurement operations for eachillumination assembly in each one of the two (or possibly even multiple)illumination sub-directions 2 a, 2 b, 3 a, 3 b and 4 a, 4 b. Accordingto a further important aspect of the invention, this can be considerablysimplified and correspondingly temporally shortened in that theweighting is performed in the measurement operation itself.

As in the known color measuring apparatus of this type, the measurementoperation is principally effected such that—separately for eachillumination channel (illumination assemblies 20, 30, 40)—by means ofthe pick-up assembly 50 a complete spectrum with a plurality of samplingpoints (wavelength ranges of e.g. each 10 . . . 20 nm width) is capturedover the wavelength range of interest (mostly visible spectrum plus nearUV). Thereto, the spectrometer 53 is activated by the control 100 for acertain time window T_(s) (set in measurement readiness) and the lightsource of the respective illumination assembly is activated or turned onfor a certain period of time within this time window. The time windowT_(s) corresponds to the integration time of the spectrometer. Invariation of this general measurement scheme, according to theinvention—again separately for each illumination channel—within the timewindow T_(s), in which the spectrometer 53 is switched to the activestate, both illumination subassemblies 20 a, 20 b and 30 a, 30 b and 40a, 40 b, respectively are each activated for an individual activationperiod of time t_(a) and t_(b), respectively (i.e. their associatedlight emitting diode light source is turned on), wherein the activationdoes not necessarily have to be simultaneously effected, but theactivation periods of time usually overlap each other. The activationperiods of time are determined according to the formula:

t _(a) =G _(i) *T ₀ and t _(b)=(1−G _(i))*T ₀

wherein G_(i) is a weighting factor from 0 to 1 and T₀ is a presetmaximum activation period of time ⇐T_(s). From the formula, it becomesimmediately clear that with a weighting factor G_(i) approaching 0, themeasurement result is increasingly determined by the illumination by theillumination subassembly 20 b and 30 b and 40 b, respectively, andconversely, with a weighting factor G_(i) approaching 1, the measurementresult is increasingly determined by the illumination by theillumination subassembly 20 a and 30 a and 40 a, respectively. Byselection of the weighting factor G_(i) or of the ratio of theactivation periods of time t_(a) and t_(b) of the two illuminationsubassemblies, thus, each arbitrary effective angular displacement canbe adjusted within the limits given by the two illuminationsub-directions. FIG. 6 illustrates these ratios. In a particularimplementation of the invention, the weighting of the period of time canalso be expanded to more than two illumination subassemblies as long asthe sum of the weights is normalized.

Basically, it is also possible to orient the illumination subassembliessuch that they do not enclose the respective nominal illuminationdirection between them, but are all on the same side next to the latter.In this case, the above outlined interpolation measures becomeextrapolation measures in analogous manner.

In FIG. 6, next to the main time window T_(s), a second, narrower(shorter in time) time window T_(s2) is also illustrated, within which asecond measurement is respectively performed under otherwise identicalconditions, wherein the activation periods of time of the twoillumination subassemblies are denoted by t_(a2) and t_(b2) and theirmutual ratio is the same as within the first time window T_(s). Themeasurements performed within this shorter time window T_(s2) arerequired for the optimum dynamic adaptation of the spectrometer 53. Inparticular in the measurement on samples with effect colors, withillumination directions near the specular direction (glancing angle),the reflectivities can become as high as the integration duration of thespectrometer must be shortened in order not to overload it. In aparticular embodiment, more than two time windows can also be employed.

In FIG. 7, the cooperation of the individual components of the colormeasuring apparatus according to the invention is illustrated in a blockdiagram. The already mentioned computer-based control 100 includes amicrocontroller 110, a hardware control stage 120, a spectrometercontrol stage 130, a data storage 140 and a USB interface 150 as themost important functional units, wherein the microcontroller 110coordinates and controls the entirety and is also concerned with thecommunication with an external computer PC connected via the USBinterface 150.

The hardware control stage 120 controls the illumination assemblies andthe illumination subassemblies 20 a, 20 b, 30 a, 30 b, 40 a, 40 bthereof, respectively, i.e. turns on and off, respectively, the lightsources contained therein. In addition, the hardware control stage 120also controls a drive 71, by which a white tile 70 can be introducedinto and again removed from the measurement beam path of the colormeasuring apparatus, respectively.

The spectrometer control stage 130 activates the spectrometer 53 andreads out the measurement data generated by it, conditions it andconverts it into digital measurement signals (spectral reflectivities).

The (non-volatile) data storage 140 substantially contains adjustmentparameters for the color measuring apparatus. In particular, thedurations of the time windows for the spectrometer and the activationperiods of time for the individual illumination subassemblies alsobelong thereto.

Before the color measuring apparatus is ready to use, it is firstcalibrated based on dark measurements and measurements on a white tile(white reference) in a manner known per se. Therein, the measurements onthe white tile are separately performed for each illumination assemblyand each illumination subassembly, respectively.

In a further step, the color measuring apparatus is profiled. Thereby,it is understood that its adjustment parameters are adjusted such thatthe color measuring apparatus provides measurement results, which matchthose of a reference color measuring apparatus as exactly as possible.The determination and adjustment of the adjustment parameters areeffected under the control of the external computer PC based oncomparison measurements. The adjustment parameters determined thereinare then stored in the data storage 140. As already mentioned, inparticular the activation periods of time of the individual illuminationsubassemblies also belong to the adjustment parameters.

The mentioned profiling can be absolute or relative. By absoluteprofiling, the adaptation to a highly precise reference color measuringapparatus (with the same measurement geometry) is to be understood. Witha relative profiling, the adjustment of the parameters is effected withregard to the adaptation to any usually not so precise or error-pronetarget color measuring apparatus (with the same measurement geometry) asexact as possible. If the illumination assemblies of the target colormeasuring apparatus are (slightly) erroneously aligned, then, the colormeasuring apparatus according to the invention can emulate thesemisalignments by corresponding choice of its adjustment parameters suchthat the color measuring apparatus according to the invention and thetarget color measuring apparatus provide (nearly) exactly the samemeasurement results. This relative profiling is e.g. advantageous ifexisting color measuring apparatus are to be supplemented or replaced bynew color measuring apparatus, but a considerable amount of measurementdata has already been generated by means of the existing color measuringapparatus and this measurement data also is to be further usable. Inthese cases, the new color measuring apparatus must provide measurementresults consistent with the existing color measuring apparatus.

As already mentioned above, the illumination assemblies 20, 30 and 40also can each have more than two illumination subassemblies. In FIG. 8,an illumination assembly 20′ with 5 illumination subassemblies 20 a, 20b, 20 c, 20 d and 20 e is for example illustrated, wherein each of theseillumination subassemblies in turn includes two light emitting diodes aslight source. The adjustment of the metrologically effectiveillumination direction is also effected in analogous manner by adequatechoice of the weighting of the individual exposures via correspondingindividual choice of the activation periods of time of the individualillumination subassemblies.

FIGS. 9 and 10 illustrate a further advantage allowed by the formationof the illumination assemblies with multiple, e.g. as shown in FIG. 8transversely to the measurement plane, with each five illuminationsubassemblies.

If only the central illumination subassembly 20 c is activated as inFIG. 9, all of the beams extend nearly parallel as shown to the right ofthe telecentric lens 21. The distribution of the luminance on themeasurement field corresponds to the curve 25′ shown rightmost in FIG.9. If all of the illumination subassemblies are activated, the beamdirections extend less collimated as shown to the right of thetelecentric lens 21 in FIG. 10. The distribution of the luminance overthe measurement spot rather corresponds to the curve 25″ shown rightmostin FIG. 10. This corresponds to an aperture stop in effect.

With the aid of the inner illumination subassemblies 20 b and 20 d, withsmall field angle as described above, the effective illuminationdirection can be adjusted (FIG. 9). With activation also of the outerillumination subassemblies 20 a and 20 e and possibly also of the centerillumination subassembly 20 c, in contrast, a larger field angle isgenerated (FIG. 10). According to number of activated illuminationsubassemblies, thereby, the aperture can be optimally adjusted.

In the illustrated embodiment, all of the three illumination assembliesare each divided in two illumination subassemblies. However, it is alsopossible to form only one or a single one of the illumination assemblieswith illumination subassemblies. This is particularly important in thatillumination assembly, which is closest to the specular direction(glancing angle), since the influence of misalignments (angular errors)is most critical near the specular direction.

A further advantage of the invention is in that by individual activationof each illumination subassembly, not only the reflection factor of asample in a certain arrangement, but also derivatives of the reflectionfactor of a sample in the direction of the illumination angle can bedetermined. This is a further criterion to differentiate materialsamples and can for example render the selection of varnish samples froma database more robust. The derivative can for example be determined byLagrange interpolation with the aid of the sampling points given by theindividual measurement values and derivative of the resulting polynomialat the desired point. The calculation of the derivative can beinternally effected by the control 100 or externally by the connectedcomputer. As a particularly advantageous characteristic of theinvention, it is not required that the illumination subassemblies aremounted with high precision, it is sufficient to be able to preciselymeasure their position relative to the nominal illumination angle.

The measurement assembly MA can also be inversely formed with respect toillumination and observation assemblies. In the specific case, thismeans that the illumination of the measurement object would only beeffected in a defined illumination direction and therefore the pick-upof the measurement light radiated back would be effected by means ofthree pick-up assemblies in three different observation directions,wherein at least one pick-up assembly would include at least two pick-upsubassemblies in analogous manner. Of course, any combinations of one ormore illumination assemblies and one or more pick-up assemblies are alsopossible. The weighting of the measurement results of the at least twopick-up subassemblies can then again be effected either computationallyor by corresponding adjustment of the activation windows (integrationperiods of time) of the pick-up subassemblies. The weighting is ofcourse separately effected for each pick-up assembly provided withpick-up subassemblies, and if two or more illumination assemblies arepresent, also individually for each of these illumination assemblies.

In FIG. 11, such an alternative embodiment of the color measuringapparatus according to the invention is schematically illustrated. Here,the color measuring apparatus includes a single illumination assembly220 and a single pick-up assembly 250, which in turn includes twopick-up subassemblies 250 a and 250 b. Each pick-up subassembly includesa light guide 252 a and 252 b, respectively, and a spectrometer 253 aand 253 b, respectively.

The illumination assembly 220 illuminates the measurement object MO inan illumination direction 202. The two pick-up subassemblies 250 a and250 b receive measurement light radiated from the measurement object MOfrom two observation directions 205 a and 205 b, respectively, whichenclose a nominal observation direction 205 between them.

Although the present invention has been described with reference toexemplary embodiments thereof, the present invention is not limited byor to such exemplary embodiments. Rather, the present invention may bemodified, refined and/or enhanced without departing from the spirit orscope of the present invention.

1.-14. (canceled)
 15. A method of providing illumination angle errorcompensation in a color measurement device, the measurement deviceincluding at least a first illumination assembly having a first nominalillumination direction with respect to a measurement spot of ameasurement object, the first illumination assembly including at leasttwo illumination subassemblies each including a light source and havingillumination sub-directions which are angularly offset to each other byan angle (Δε) of 4°-8° and enclosing the first nominal illuminationdirection between them, and a pick-up assembly for capturing themeasurement light radiated back from the measurement spot along anobservation direction and for converting the same into correspondingelectrical signals, said method comprising profiling the measurementdevice by: performing separate calibration measurements of a referencemeasurement object with each illumination subassembly providingillumination at their respective actual illumination sub-directions;determining adjustment parameters for the first illumination assemblyfrom the calibration measurements such that measurements made by thecolor measurement device coincide with reference measurements made withillumination at the first nominal illumination direction; and storingthe adjustment parameters in data storage of the color measuring device.16. The method of claim 15, wherein the adjustment parameters compriseweighted activation durations for each subassembly.
 17. The method ofclaim 16, wherein the step of determining adjustment parameters furthercomprises determining the weighted activation duration for eachillumination subassembly by multiplying a predetermined maximum durationof illumination by a weighting factor for each illumination subassembly.18. The method of claim 17, further comprising the steps of: activatingthe pick-up assembly for a time window; during the time window,individually controlling the activation of the illuminationsubassemblies in accordance with the weighted illumination durations;and obtaining a weighted illumination measurement from the at least onepick-up assembly measurement including two or more illuminationsub-directions of the first illumination assembly.
 19. The method ofclaim 15, wherein the adjustment parameters comprise measurement summingweighting factors.
 20. The method of claim 19, further comprising thesteps of: making a first measurement by activating the pick-up assemblyfor a first time window and individually controlling the activation of afirst illumination subassembly within the first time window; making asecond measurement by activating the pick-up assembly for a second timewindow and individually controlling the activation of a secondillumination subassembly within the second time window, the first andsecond illumination subassemblies being part of the first illuminationassembly; weighting each measurement according to each illumination subassembly's measurement summing weighting factor; and summing theweighted measurements.
 21. The method of claim 15, wherein themeasurement device further comprises at least one second illuminationassembly being oriented with a second nominal illumination directionwith respect to the measurement spot, wherein the second nominalillumination direction differs from the first nominal illuminationdirection by at least 15 degrees, and wherein the second illuminationassembly includes at least two illumination subassemblies whichilluminate the measurement spot from illumination sub-directions whichare angularly offset to each other by an angle (Δε) of 4°-8° and enclosethe second nominal illumination direction between them.
 22. The methodof claim 21, wherein the steps of the method of claim 15 are repeatedfor the second illumination assembly.
 23. The color measuring apparatusaccording to claim 15, further comprising an electronic controlconfigured with instructions to operate the illumination subassembliesand the pick-up assembly.
 24. The method of claim 15, wherein theillumination sub-directions for an illumination subassembly are in acommon measurement plane with respect to the observation direction. 25.The method of claim 15, wherein the step of determining adjustmentparameters is repeated for a plurality of wavelengths of light.
 26. Themethod of claim 15, wherein the first nominal illumination direction isestablished by a reference color measurement apparatus comprising atleast one illumination assembly and at least one observation assembly,wherein the at least one illumination assembly and at least oneobservation assembly have the same measurement geometry as the colormeasurement device.
 27. The method of claim 26, wherein the step ofprofiling the measurement device comprises relative profiling.
 28. Themethod of claim 26, wherein the step of profiling the measurement devicecomprises absolute profiling.
 29. A method of making a measurement withillumination angle error compensation in a color measurement device, themeasurement device including at least one illumination assembly having anominal illumination direction with respect to a measurement spot of ameasurement object, the illumination assembly including at least twoillumination subassemblies each including a light source and havingillumination sub-directions which are angularly offset to each other byan angle (Δε) of 4°-8° and enclosing the nominal illumination directionbetween them, a pick-up assembly for capturing the measurement lightradiated back from the measurement spot along an observation directionand for converting the same into corresponding electrical signals, andadjustment parameters for the illumination subassemblies stored in adata storage, the adjustment parameters comprising individual activationdurations for each subassembly weighted to provide illumination for aweighted illumination measurement that corrects for the differencebetween the actual illumination angle and the nominal illumination anglesaid method comprising: activating the pick-up assembly for a first timewindow; individually controlling activation durations of at least two ofthe illumination subassemblies within one illumination assembly inaccordance with the adjustment parameters and within the first timewindow; and obtaining a first weighted illumination measurement from thepick-up assembly measurement including two or more illuminationsub-directions of an illumination assembly.
 30. The method of claim 29,wherein the individually controlled activation durations overlap intime.
 31. The method of claim 29, wherein the steps are repeated foreach illumination assembly.
 32. The method of claim 29, wherein the timewindow corresponds to the integration time of the spectrometer.
 33. Themethod of claim 29, further comprising the steps of: activating thepick-up assembly for a second time window, the second time window beingof different duration than the first time window; individuallycontrolling activation durations of at least two of the illuminationsubassemblies within one illumination assembly in accordance with theadjustment parameters and within the second time window; and obtaining asecond weighted illumination measurement from the at least one pick-upassembly measurement from two or more illumination sub-directions of anillumination assembly.
 34. The method of claim 33, wherein the secondtime window has shorter duration than the first time window.
 35. Amethod of making a measurement with illumination angle errorcompensation in a color measurement device, the measurement deviceincluding at least one illumination assembly, the illumination assemblyincluding at least two illumination subassemblies each including a lightsource, the illumination assembly being oriented with an actualillumination direction with respect to a measurement spot of ameasurement object and a nominal illumination direction, eachillumination subassembly illuminating the measurement spot fromillumination sub-directions that differ from the actual illuminationdirection by angles of no more than 4° and having a measurementsummation weighting factor to correct for the difference between theactual illumination angle and the nominal illumination angle, a pick-upassembly for capturing the measurement light radiated back from themeasurement spot along an observation direction and for converting thesame into corresponding electrical signals, said method comprising:making a first measurement by activating the pick-up assembly for afirst time window and individually controlling an activation of a firstillumination subassembly within the first time window; making a secondmeasurement by activating the pick-up assembly for a second time windowand individually controlling an activation of a second illuminationsubassembly within the second time window; weighting each measurementaccording to each sub assembly's weighting factor; and summing theweighted measurements to obtain a measurement corrected to the nominalillumination angle.
 36. The method of claim 35 wherein at least twoillumination subassemblies comprises a first illumination subassemblyhaving a first weighting factor g and second illumination subassemblyhaving a second weighting factor 1-g.
 37. The method of claim 35,wherein the steps are repeated for a plurality of wavelengths of light.